NANCY WALKER thinks she has a solution for The Guest Who Wouldn't Leave.

If you have a visitor who tends to stay around for six-week stretches, Walker says, "The best thing is to paint lots of vibrant colors in the room. That will drive her out in about three days."

Walker comes from Riverside, Ill., a suburb of Chicago, where they make no bones about disliking unwelcome visitors. She is also president of the National Home Fashions League and a recognized expert in the use of color.

Walker was in Washington a couple of weeks ago to address members of the local league chapter. The subject of her talk was the use of color in "communications and marketing." Walker is one of those who believes, and who is paid to show, that it's not always what you see that you get. Sometimes it's what you didn't see that made you buy it.

For instance, colors in a department store should be as pleasant yet as unobtrusive as possible, setting off, very neatly, the products on display.

"Color, being a visual experience," says Walker, "is the trigger that makes people buy. They may walk into a store not intending to buy anything. But they may forsake price and quality if the color setting is right . . . Good merchandisers know the value of color and light."

Walker also believes colors can have therapeutic qualities.

"It's been found," she says, "that hospital patients actually recovered more quickly when the rooms were painted yellow. A cheerful kind of environment reminds you of sunshine, reminds you of happy things.

"The best way to cheer yourself up," she says, "is to surround yourself with cheerful colors."

(We can infer, then, that it is not wise to paint your guest room yellow. Or even apricot, for that matter.)

Walker bases her comments on nearly 20 years in the business. Her first job was with Martin Senour paints in the early '60s. Now she runs the firm of Color Counselors, in Chicago, and tells companies such as Brunswick, which makes bowling equipment, how to get women to buy Brunswick bowling balls, Brunswick bowling shoes and Brunswick bowling bags.

You make the colors match.

That may sound simple. But color, like fashion, is always changing. If you want to remain in business, you have to stay on top of things. "There's sort of a network of designers-and the designers, along with the magazine editors, have a great deal of influence on what gets into the stores."

Walker has seen pink and turquoise come and go. She's witnessed the birth and demise of coppertone. In her day, avocado rose like a conquering general, only to find its Waterloo in the kitchens of Kalamazoo.

"Color, like designs in interiors," she says, "is in a cycle. When we get as dark as we can go, we lighten up again."

Now it's "earth colors" or the "neutral look"-beiges, browns, deep chocolates, grays, "greiges" and charcoals. Walker says these colors are "tied very directly into a casual type of lifestyle. The home is the last place a person can make into an extension of his personality."

Matching colors and furniture are out. Laid-back colors and shiny accessories are in.

For today's generation, "one piece becomes the focal point. Everything else (including color) is chosen around it. The piece may not be necessarily expensive, nor necessarily matching. People want to be individuals. They don't want their room looking like everybody else's."

Walker has a few tips for consumers:

If you're working with an interior designer, he should be ready to listen to what you want. "Any good interior designer is one that designs for his customers, not for himself."

Be wary of matching colors with oft-used color names. "There's no standardization of color names. Golds run the gamut. You can waste a lot of time looking for the true beige of the true antique white. You always have to have a point of reference. Take samples of what you have. You just have to push around a lot of color chips, as I did."

If you don't know where to start with your color scheme, start with the colors you want to highlight most. "Start planning around anything with a pattern-rug, draperies, wallpapers or a painting. Select the colors from the pattern and draw them out."

It is a myth, says Walker, that white ceilings are necessary to reflect light. "That's a big fallacy. Unless you beam a light directly on the ceiling, it's always in full shadow. You can continue light colors right up from the wall."

Be careful when using light colors on all four walls. "Most people don't realize that with warm colors, such as yellow, you have four walls reflecting on each other. It's best to select a shade paler."

One color, if painted on each wall and the floor, may not appear the same everywhere. "A lot of people like to continue the floor color onto the walls. The wall color should be a shade lighter in value (but the same hue), because the side walls are in half shadow." CAPTION: Picture, Color consultant Nancy Walker: "When we get as dark as we can go, we lighten up again."